Process for production of glass preform for optical fiber and apparatus for the same

Abstract: A process for the production of a porous glass preform for an optical fiber and an apparatus for such a process are provided. In the process, fine glass particles are produced by heating and reacting vaporous starting materials for glass in a reaction vessel, the thus produced fine glass particles are deposited to obtain the porous glass preform and an exhaust gas speed through an exhaust duct which exhausts an exhaust gas containing non-deposited fine glass particles from the reaction vessel is controlled to be above 15 m/sec. Preferably, pressure fluctuation in the reaction vessel is minimized by supplying air from the outside of the reaction vessel to the exhaust duct. According to the present invention, the exhaust duct is not likely to be blocked by the non-deposited fine glass particles so that the stable production of the preform is maintained over an extended period.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for the production of a glass preform for an optical fiber and an apparatus for carrying out such a process. Particularly, the present invention relates to a process for the production of a porous glass preform in which non-deposited fine glass particles are prevented from their deposition to the inside of an exhaust duct and/or their residence within the exhaust duct when the glass preform is produced by deposition of fine glass particles, and also relates to an apparatus for such a process. Description of the Related Art As a process for the production of a glass preform for an optical fiber, a process using vapor phase reactions has been known. As an example of such a process, the inside chemical vapor deposition (CVD) process, the outside chemical vapor deposition process and the vapor phase axial deposition process are known. In these processes, a vaporous starting material for glass flows out through an oxyhydrogen burner nozzle and a flame hydrolysis reaction occurs there, whereby fine glass particles produced from the reaction are deposited in the form of a rod to obtain the porous glass preform for the optical fiber in a reaction vessel. Fig. 2 schematically shows a prior art system for the production of the glass preform for the optical fiber. In such a system, the fine glass particles are produced from combustion of a starting material gas of glass such as SiCl 4 gas, hydrogen gas and oxygen gas each flowing out of the burner 2 in the reaction vessel 1, and the particles are deposited on the seed rod 4 as the porous glass preform 3. The seed rod 4 is grasped and rotated and also lifted by the chuck means 5 as the fine glass particles are deposited thereon. In the process just described above, reactions occur according to the following reaction formulas in the reaction vessel in the case in which SiCl 4 is used as the starting gas material of glass: H 2 + 1/2 O 2 → H 2 O + 57.8 Kcal/mol SiCl 4 + 2 H 2 O → SiO 2 + 4 HCl As seen from the above formulas, heat of formation is generated due to the reaction of hydrogen and oxygen together with formation of SiO 2 fine particles. Usually, about 50 to 90 % by weight of the fine glass particles produced according to the above reactions (SiO 2 fine particles in this case) is deposited in the form of a column to produce the porous glass preform, and the balance is not deposited and exhausted through a outlet port 6 to the outside of the reaction vessel 1 by being entrained with an exhaust gas. Such fine glass particles which have not been deposited to form the preform in the reaction vessel will be, hereinafter, referred to as "non-deposited (fine) glass particles". The exhaust gas comprises, in addition to the non-deposited fine glass particles, the product gas materials (such as H 2 O, SiO 2 and HCl), and also comprises ambient air around the reaction vessel which comes from the suction port of the reaction vessel. The exhaust gas containing the non-deposited glass particles reaches the fine glass particle scrubber 9 through a connection pipe 7 and an exhaust duct 8, and the gas is separated from the fine glass particles in the scrubber 9. Then, the exhaust gas free from the glass particles is withdrawn from the system through the acid gas treatment facility 10 to the ambient atmosphere (usually to the outside of a clean room in which the reaction vessel 1 is placed). It should be noted that some fine glass particles remain in the reaction vessel 1 which attach to the inside surface of the reaction vessel, but a ratio of such particles to the formed particles is very small. As to the heat of formation due to the reaction of oxygen and hydrogen, usually about 60 to 90 % thereof is exhausted with the exhausted gas so that the exhaust gas is heated to an elevated temperature. It is well-known that the apparatus for the production of the glass preform for the optical fiber is placed in a clean room which is pressurized positively so as to keep the cleanness in the room. In order to keep the positive pressure in the clean room, an amount of air exhausted from the clean room should be controlled to a limited certain amount. Thus, it is generally of course that the apparatus for the production of the glass preform for the optical fiber is so arranged as to minimize an amount of the exhaust gas which is withdrawn from the reaction vessel because the exhaust gas comprises the ambient air of the reaction vessel and is drawn out to the outside of the clean room. Since, for such a purpose of minimizing the amount of the exhaust gas, an exhaust gas speed through the exhaust duct is as low as about 7 ± 3 m/sec., fine glass particles which have not been deposited on the rod (i.e. the non-deposited fine glass particles) are very likely to deposit on the inside surface of the duct and finally the duct is likely to be blocked with the deposition of the particles. Due to the possibility of the duct blockage, operation of the apparatus for the production of the glass preform for the optical fiber, hitherto, has to be stopped once per every two or three months and the inside of the duct should be cleaned. Therefore, the productivity of the glass preform is reduced. Also, the blockage tendency of the duct makes an operation pressure in the reaction vessel unstable so that stable production becomes impossible. In addition, materials of which duct is formed are heated to an elevated temperature due to the heat of formation of the reaction between hydrogen and oxygen, and normal materials for the duct cannot withstand such an elevated temperature. Therefore, the duct should be made of special materials which is generally expensive such as a heat resisting glass, Teflon and a high grade metal (for example, pure nickel, a nickel based alloy and so on). JP-A-59128226 (Patent Abstracts of Japan, Volume 8, No. 246 (C-251) 1984) refers to a controlling method of exhaust pressure of a VAD-reaction vessel; according to this method a control valve is provided in the air supply line to the exhaust line in order to stably carry out the manufacture of the base material for an optical fiber without any trouble in the reaction vessel. JP-A-60041539 (Patent Abstracts of Japan, Volume 9, No. 171 (C-291) 1985) refers to an apparatus for removing automatically deposited products in order to prevent clogging of a discharge pipe by excessive reaction product by providing small holes to the discharge pipe for the excessive reaction product. SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for the production of a glass preform for an optical fiber which overcomes the problems just described above. Subject matter of the present invention is a process for the production of a glass preform for an optical fiber according to Claim 1. In the present invention air available from the outside of the reaction vessel is supplied to the exhaust gas leaving the reaction vessel at a position of the exhaust duct immediately downstream of the reaction vessel so that the above specified exhaust gas speed is achieved. In the present invention, it is preferably that pressure fluctuation in the reaction vessel is minimized by supplying air from the outside of the reaction vessel to the exhaust duct through which the exhaust gas containing the non-deposited fine glass particles is exhausted from the reaction vessel. Further subject matter is an apparatus for the production of a porous glass preform for an optical fiber according to Claim 2. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows a system for the production of the porous glass preform for the optical fiber which comprises one embodiment of the apparatus according to the present invention; Fig. 2 schematically shows a system of the prior art for the production of the porous glass preform for the optical fiber; and Fig. 3 shows a graph which indicates a relationship between a cleaning interval (month) and an exhaust gas speed (m/sec.). DETAILED DESCRIPTION OF THE INVENTION Hitherto, it has been said that it is very difficult to prevent deposition of the non-deposited fine glass particles within the exhaust duct through which the exhaust gas containing the non-deposited fine glass particles is withdrawn to the outside of the reaction vessel. However, the present inventors have found that the deposition and/or the residence in the exhaust duct of the fine glass particles which are entrained in the exhaust gas is prevented so that the exhaust duct is not likely to be blocked provided that the exhaust gas speed through the exhaust duct is above 15 m/sec. and preferably above 20 m/sec. As described above, the exhaust gas speed through the exhaust duct has been 7 ± 3 m/sec. in the prior art process since the clean room in which the reaction vessel is placed should be kept positively pressurized and thus the exhaust gas from the reaction vessel should be minimized. In the present invention, a large amount of air may be supplied to the exhaust duct in order to increase the exhaust gas speed. However, it should be noted that, in the present invention, the air in the clean room is not added to the duct intentionally through the reaction vessel to have the increased speed. To the contrary, the prior art process, the air is supplied through the suction port of the reaction vessel, and then exhausted from the reaction vessel. In the present invention, the supplied air is preferably available from the outside of the reaction vessel (i.e. ambient air around the reaction vessel) or from the outside of the clean room. The latter is more preferable because there is substantially no effect on the positive pressure in the clean room even when a large amount of the air is supplied to the exhaust duct. In the present invention, the air is supplied directly to a connection pipe which is located just downstream of the reaction vessel, whereby the positive pressure in the clean room is ensured while the exhaust gas speed is increases to as much as above 15 m/sec. As a result, the blockage of the exhaust duct can be easily prevented over an extended period. According to the present invention, the operating pressure in the reaction vessel becomes stable so that the productivity of the glass preform for the optical fiber is greatly improved. There is no specific upper limit of the exhaust gas speed within the exhaust duct. However, when the speed is excessively higher, apparatuses which treat the exhaust gas would be larger in their scales and thus their costs would be increased. From these considerations, the upper limit of the exhaust gas speed may be practically determined. The speed of the exhaust gas through the duct is more preferably above 20 m/sec. Most preferably, the gas speed is above 20 m/sec. on the average. This is because the exhaust gas speed of at least 15 m/sec. is almost always ensured even though the speed is sometimes lowered to below 20 m/sec. temporally. In addition, the exhaust gas which exits the reaction vessel at an elevated temperature is diluted and thus its temperature is reduced by supplying a large amount of the outside air to the exhaust duct. Thus, the duct may be made of a normal duct material such as a heat resisting polyvinyl chloride resin or a fiber reinforced plastic (FRP) so that costs of the exhaust duct and its following apparatuses are reduced. DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus for the production of the glass preform for the optical fiber according to the present invention will be, hereinafter, explained in detail with reference to the accompanying drawings. Fig. 1 schematically shows one embodiment of the present invention in which the same numerals as in those of Fig. 2 indicate the same members as those of Fig. 2. The numeral 1 indicates a reaction vessel, 2 does a burner, 3 does a porous glass preform, 4 does a seed rod, 5 does a chuck means, 6 does an exhaust port, 7 does a connection pipe, 8 does an exhaust duct, 9 does a scrubber for the fine glass particles, 10 does an acid gas treatment facility, 11 does an exhaust fan, 12 does an outside air supplying conduit and 13 does a valve which controls the introduction of the outside air. The alternate long and short dash line in Fig. 1 (and Fig. 2) indicates a clean room in which the reaction vessel 1 is placed. Although the number of the reaction vessel 1 in the clean room is not specially limited, in Fig. 1 (and Fig. 2), only two sets of the reaction vessel (the first reaction vessel and the n-th reaction vessel) are shown. The reaction vessel 1 is provided with the exhaust port 6 through which the exhaust gas containing non-deposited fine glass particles is withdrawn from the reaction vessel 1. The exhaust gas containing the non-deposited particles is introduced through the connection pipe 7 to the exhaust duct 8. The outside air is supplied from the inside of the clean room or from the outside of the clean room to the connection pipe 7 through the outside air introducing tube 12 and the control valve 13 which controls the amount of the supplied outside air so as to keep the exhaust gas speed through the duct 8 above 15 m/sec., and preferably above 20 m/sec. on the average. Usually, a plurality of the apparatus for the production of the glass preform for the optical fiber are placed in one clean room as described above, and the exhaust gas is exhausted from each reaction vessel 1 to the duct 8. The exhaust gas is gathered through the duct 8 into the scrubber for the fine glass particles 9 where the non-deposited fine glass particles are removed. The exhausted gas is then treated in the acid gas treatment facility 10 so that HCl gas contained therein is removed from the exhaust gas (mostly air), which is now harmless and which is exhausted to the ambient air by means of the exhaust fan 11. As the scrubber 9 of the non-deposited fine glass particles, for example, an electrostatic precipitator or a Venturi scrubber may be used. In the acid gas treatment facility 10, it is conventional to shower a sodium hydroxide aqueous solution for neutralization of HCl. EXAMPLE Although the present invention will be further described with reference to the following example, the present invention is not limited to this example. Example 1 Using the reaction vessel system as shown in Fig. 1 according to the present invention, the porous glass preforms for the optical fiber were produced. In this example, an amount of the exhaust gas from each reaction vessel 1 was kept constant for the stable production operation of the preforms while the exhaust gas speed through the duct 8 was controlled above 15 m/sec. on the average by using the outside air control valves 13. The production of the glass preforms was continued over more than eighteen months, and no blockage due to the non-deposited glass particles happened and also there was no need for the cleaning of the duct during the continued production, so that the productivity of the preform was remarkably improved. Compared with the prior art process in which the cleaning of the duct has been carried out at an interval of twice or three times per month, the effect of the present invention was confirmed in this example. Fig. 3 shows a graph indicating a relationship between the exhaust gas speed and the interval between the successive cleanings of the duct. On the basis of data of average amounts of a hydrogen flow rate and an exhaust gas flow rate of Example 1, a temperature of the exhaust gas flowing through the duct 8 was obtained and shown in the following Table 1 together with the data of the prior art process. In Table 1, the duct materials which can be used in the processes are also shown. Present Invention Prior Art Hydrogen flow rate 80 liter/min. 80 liter/min. Exhaust gas flow rate 10 m 3 /hr 3 m 3 /hr Exhaust gas temperature in duct 80° C 210°C Duct material ∘ Heat resisting polyvinyl chloride ∘ Glass ∘ Teflon ∘ FRP ∘ High grade metal It is seen from the above Table 1 that, in the present invention, the exhaust gas temperature in the exhaust duct is considerably lower and the exhaust duct can be, therefore, made of cheap materials such as the heat resisting polyvinyl chloride and FRP. In the prior art, PYREX® glass is used for the exhaust duct. As seen from the above description, since, in the apparatus for the production of the glass preform for the optical fiber according to the present invention is different from the apparatus of the prior art in that it is possible for the former to increase the exhaust gas speed by supplying the outside air to the exhaust duct immediately downstream of the reaction vessel independently of the maintenance of the cleanness in the clean room, not only the temperature of the exhaust gas can be reduced but also the exhaust gas speed through the exhaust duct is increased to above 15 m/sec., so that no blockage of the duct is likely to occur due to the non-deposited fine glass particles, and the inner pressure of the reaction vessel can be, thus, kept stable for an extended period. Therefore, the productivity of the glass preform of the optical fiber is improved.

Claims: A process for the production of a porous glass preform for an optical fiber in which fine glass particles are produced by heating and reacting vaporous starting materials for glass in a reaction vessel and the produced fine glass particles are deposited to obtain the porous glass preform, characterized in that air from outside the reaction vessel is supplied to the gas containing non-deposited fine glass particles, which is leaving the reaction vessel, at a position of the exhaust duct immediately downstream of the reaction vessel and that the exhaust gas speed through the exhaust duct is controlled to be above 15 m/sec. An apparatus for the production of a porous glass preform for an optical fiber comprising a reaction vessel (1) in which starting materials for glass are reacted to produce fine glass particles which are deposited as the porous glass preform (3) characterized in that the apparatus comprises a means (12) which supplies air from the outside of a room in which the reaction vessel is located to an exhaust duct (8) through which an exhaust gas containing non-deposited fine glass particles is drawn out of the reaction vessel, and in, that it comprises control valve (13) which controls the amount of the supplied outside air so as to keep the exhaust gas speed through the duct (8) above 15 m/sec.

Inventors: AMEMIYA KOJI, HABASAKI TOSHIMI, TAKAHASHI YUJI, TAKATA KOJI, UCHINO FUMITAKA, AMEMIYA, KOJI, HABASAKI, TOSHIMI, TAKAHASHI, YUJI, TAKATA, KOJI, UCHINO, FUMITAKA, AMEMIYA, KOJI, C/O YOKOHAMA WORKS, HABASAKI, TOSHIMI, C/O YOKOHAMA WORKS, Takahashi, Yuji, c/o Yokohama Works, TAKATA, KOJI, C/O YOKOHAMA WORKS, UCHINO, FUMITAKA, C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1994
Main classification: C03B37
IPC codes: C03B37, G02B6, C03B8, C03B20